Optical device for testing crystal sections



April 29, 1947. 5. w. WILLARD OPTICAL DEVICE FOR TESTING CRYSTAL SECTIONS Original Filed Sept.

9, 1943 3 Sheets-Sheet l INVENTOR By 6.! WILLARD v 3 S R Q a. A, d. NR

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A TTORNEV G. W. WILLARD OPTI CAL DEVICE FOR TESTING CRYSTAL SECTIONS April 29, 194?.

Original Fi led Sept. 9, 1943 5 Sheets-Sheet 2 FIG. 4

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INVENTOR aw. WILLARD 8V 6 v Q ATTORNEY April 29, 1947. G. w. WILLARD 2,419,517

OPTICAL DEVICE FOR TESTING CRYSTAL SECTIONS I 3 Sheets-Sheet 3 7 FIG. /6

V o M RR R m m a M T 1 L A "P M L MU WP M W 2 2H 2 V B W 31mm mp m 5 MP u u. 0 mi H m a w R w w M F F F Patented Apr. 29, 1947 UNITED STATE-S PATENT OFFICE Gerald W; Willard, Fanwood, N."J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original applicationseptember 1 943, SerialNo. 501,619. Divided and this application October 12, 1944, Serial No. 558,336

This invention relates to an optical device and particularly to an optical device for the examine, tion or testing of quartz piezoelectric specimens.

An object of the invention is to facilitate the production of quartz piezoelectric plates.

A more specific objectofthe invention is to facilitate the examination and testing of sections or other portions of quartz crystals for determination of the direction and sense of one or more of the natural crystallographic axes or for the detection of twinning,

This application is a division of my application Serial No. 501,619, filed September 9, 1943, entitled Optical device.

Raw quartz, from which quartz piezoelectric plates of the type now being used in increasing numbers in radio and other electrical circuits are produced, haS, ,as is Well known, three sets of crystallographic .ax'es commonly known as the X or electricalaxes, the Y or mechanical axes;

and the'Z or optical axis, respectively. The piezorespect toone or. more of thenatur'al axes.. fFor example 'a plate may be cut from ,the'raw' quartz crystal so orientated that the. major, surfaces of the plate are parallel to an electrical axis .of the crystal and inclinedla definite amount andiri the proper sense with respect to theop tical axis. It willjb'e apparent, therefore, thatat certain points in the production of quartzpiezoelectric plates it may become necessary to determinethe direc tion and sense of certain of the natural axes of the portion of quartz being-used. Further, since two varieties of twinning, i. e., electrical and optical, are commonly found in quartz and since the twinning aifects the sense, but not direction, of the crystallographic axes, it is also necessary during the cutting process to jdetect the twin--v ning, ifit exists, and to locate theboundaries between twins wherebythe quartz piece to be cut may be marked off into separate homogeneous regions. The arrangement of the present invention'permits the ready detection of the presence o'ftwinned regions and'loc'ation off th'e'bounda'ries thereof; Afterthe performance of this step the arranlgement then permits the determination, in oneregion'at a'ti'me, of thedirections and sensesoi axes or cuts to bemade therefrom, or the sense o ff'euts alreadyma'cle therefrom, or, the electrical {sense or opticalhandedne'ss of quartz specimens. The arrangement ef the; present invention; in one embodiment; permits the direction of'the X axes of a 2 cut quartz 'secti on to'be' readily gi'Clairn. (CI. 88-14) determined as wellas the electrical sense (polarity) of each X axis. This information together with a knowledge of the handedness of the portion (also may be determined by use of the arrangement of the present invention) may determine the direction and sense of angular displacement fromthe optical axis of plates to be cut therefrom.

Inaccordan'ce with another embodiment, the arrangement of the present invention permits the direction'of the Z axis of an X cut quartz section to be readily determined as well as the direction and sense, i. e., the angular displacement from the'optical axis, of plates to be cut therefrom. As used herein, Zcut section refers to a section so out from the raw quartz crystal, commonly referred to as the mother crystal that the major surfaces of the section are perpendicular to the Z. axis of the crystal while X cut section refers to a section so out that the major surfaces are perpendicular to an X axis of the crystal. A section is ordinarily fromiyz inch to 2 inches in thickness and is usually cut into slabs (relatively thin portions Whose thickness is somewhat greater than that of the desired finished plate) or bars (relatively long and narrow portions" whose smallest cross-section is somewhat greater than the major face dimensions of the finished plate). The slabs are diced into the final wafers or plates while the bars are loafed or sliced into such plates.

The quartz specimen is normally first examined for twinning by use of the arrangement contemplated by the invention as pointed out above and marked oif into homogeneous regions sufficiently large for practical use; it is only such usable regions'that arenormally examined subsequently for direction and sense of crystallographic axes or desired cuts etc.

A feature ofthe present invention is the loca- I tion of alight diffusing element closely adjacent toan illuminated pin-hole and remote from the actual primary source of light; the illuminated pin hole acts as a secondary source of light for the quartz specimens being examined.

Another feature of the invention is an arrangementwhereby the light diffuser, lens assembly andother elements are readily removable f or cleaning and are easily replaceable.

In accordance with a specific embodiment of the invention; acry'stal section supporting-table isprovided havln gfan aperture of pin hole di I mensions passing "therethrough, the crystal specimen to'beexamined being supported on the table over the pin hole aperture. A source of light is provided together with an optical system for concentrating the rays of light onto the light diffusing element and thence through the pin hole into the quartz section. At least onemajor surface of the crystal section to be observed should be previously prepared by etching, for

example by use of hydrofluoric acid. As the observer views the illuminated pin hole through the upper etched surface of the quartz section he will see a figure which appears to belocated down in the section (as when looking at objects under water). If the portion of the section being examined is untwinned, the figure will be, roughly, a three pointed star and its orientation on the section is definitely related to, and. therefore will be an indication of, the direction'and the sense of the three electrical axes of the section. Two viewing screens carried by flexible arms are provided (one for right-hand quartz and the other for left-hand quartz) and the proper screen is brought into position and the quartz section is rotated the required amount to bring the figure into alignment with guide lines provided on the respective screens. A. marking template also carried by a flexible arm is next brought into position and the directionand sense of the electrical axes are marked on the section in accordance with guides provided by the template.

Complete understanding of the arrangement and operation. of the devices contemplated by the present invention may be gained from consideration of the following detailed description and the accompanying drawings in which:

Fig. 1 is a side elevational view of an optical testing device embodying features of the present invention, a portion of the side walls being broken away to reveal the interior arrangement;

Fig. 2'is a plan view of the device of Fig. 1;

Fig.3 shows the relative positions of one arrangement of the matching and marking arms and shows the matching arm for one handedness of quartz in operative position;

Fig. 4 shows the marking arm of the first arrangement;

Fig. 5 is a viewsimilar to that of Fig. 3 except for the fact that the matching arm for the opposite handedness of quartz is shown in operative position;

Fig. 6 shows the type of figure seen in a Z cut crystal section when examined in the apparatus of Fig. 1, the three guide lines of the matching arm being indicated by broken lines; v

Fig. 7 shows the section illustrated in Fig.6 after rotation to bring the figure into alignment with the guide lines of the matching arm;

Fig. 8 shows the crystal section after marking to indicate the X axes by use of the marking arm; I

Fig. 9 is a view from below of the arm provided for holding the light diffusing element in position, the normal position of the elementwith respect to the aperture in the arm being indicated by dotted lines; 1

Fig. 10 shows one marking arm of a second arrangement; Fig. 11 shows the matching arm of said second arrangement;

Fig. 12 shows the second marking arm of the second arrangement; 1

Fig. 13 shows a crystal section after marking to indicate the direction of the Z axis and the-sense of cut for a standard crystal .platey Fig. 14 shows the type offiguref seen in a Z cut crystal section of right-handed quartz in the event the etching process has been continued for a comparatively long period before examination of the section;

Fig. 15 shows the type of figure seen in the event of section of Fig. 14 had been left-handed quartz;

. Fig. 16 illustrates a hypothetical Z out section of quartz and shows the relationship of the figures to the sense of the X axes and to the handedness of the quartz;

Fig. 17 is similar to Fig. 16 but shows the types of figures occurring when the etching process has. been continued for a comparatively long period of time;

'- Figs. 18, 19 and 20 show alternate methods of mounting a specimen on the table; and

Fig. 21 shows the relationship between the different types of figures and the four possible different types of twinning in an X cut quartz section.

Referring now to the drawings and first to Figs. 1 to 5, inclusive. there is illustrated a rectangular base plate ll upon which are mounted lamp housing l3, which is a box-like enclosure, and lens and mirror assembly housing [4, which is also a box-like enclosure. Housing I4 is provided with a hinged cover which, when closed, forms a sloping support or table designated [5.

It will be understood'that base plate I l, which is shown broken away, may be extended to support a power transformer or other associated equipment. Housings l3 and 14, which are slightly separated so that the heat from the ventilated lamp housing I 3 may be isolated from the optical housing [4, together form a substantially light impervious optical system when table I5 is in.

closed position.

Three arms, matching arms 2! andv 22 and marking arm 23, are mounted on table l5, each arm being hinged and the relative positions of the three being such that any one may be swung down into front or, operative position while the othertwo are in rear or non-operative position wherein they do not interfere with the use of the operative arm; in Figs. 1, Z and 5 arm 22 is shown in operative position; in Fig. 3 arm 2| is shown in' operative position and in Fig. 4 marking arm 23 is shown in operative. position.

Aperture 24, which is of pin hole proportions, is provided in the upper surface of table l5; this small aperture is enlarged in the lower surface of the table sufficiently to contain disk 25. Disk 25, which acts as a light diffuser element, may comprise a disk of optical glass with an enameled face. (This is often referred to as flashed glass) The enameled face of the disk is placed up,i. e., toward the aperture 24. As shown in Fig.1, the upper defining wall of the larger por tion of aperture 24 is tapered whereby only the peripheral'portion of difiuser disk 25 is contacted thereby and a space is left between the central surface'of the disk and the aperture. This space serves two. main purposes, first, it permits a .wide angle of illumination through the pinhole and, second, it provides a reservoir wherein dirt may accumulate so that the unit need be cleaned less frequently than would be the case otherwise.

Lamp 4| is mounted in lamp housing I3, being supported by bracket42. The two screws clamping bracket 42 to.housing 13 pass through-holes which are considerably oversize whereby the bracket maybe moved about to accurately focus lamp 4|. with respect to pin hole aperture 24. The arrangement is such also that the lamp may be readily removed for cleaning or replacement. Bracket 43 is mounted in the upper portion of lamp housing I3, the longer arm thereof extending out between lamp ll and the ventilation hole in the top of the housing and acting as a light shield.

Lamp 4| is provided with suitable conductors 44 and 45 (shown in part only) for connection to the energizing circuit which may include a power transformer, preferably supported on an extension of base II, a suitable control switch and other associated parts.

Two plano-convex lenses, 45 and 46, are positioned in lens assembly housing I4. Each lens is provided with a lens holder (holder 41 provided for lens 45 and holder 5| for lens 46) and the two holders are so held together by means of bolts 52 and 53 and face plates 54 and 55 that a compact and unitary assembly results, lenses 45 and 46 being held with convex surfaces'toward each other. The lens assembly is normally held in the position illustrated in Fig. 1 by side strips 55 and 5'! and two similar, but oppositely located, side strips that are not shown and by bottom strip H and asimilar, oppositely located, bottom strip not shown. The assembly may be readily removed, however, by lifting it up out of the holding strips after first raising table I5; after removal of the lens assembly the outer surfaces of the lenses may be readily cleaned.

. Light reflecting mirror 12 is angularly positioned in lens assembly housing I4, being supported on bracket l3 and by strip I4 and a similar, but oppositely positioned, strip that is not shown. Stop member 15 mounted on the lower edge of bracket I3 prevents the mirror from sliding downward along the surface of the bracket while spring member I6 normally prevents retraction of the mirror along the surface of the supporting bracket. However, the mirror may be readily removed for cleaning 'or replacement by first moving table I5 to open position and then pressing down the free end of spring 16 a suflicient amount to permit the mirror to clear the edge of the spring as it is retracted.

Light diffusing disk 25 is normally held in the position illustrated in Fig. 1, i. e., in theenlarged aperture beneath pin hole 24, by arm 8I which is rotatably attached to the under surface of table I 5 by screw 82. Arm BI is provided with aperture- 83 which normally permits passage of light to disk 25. Aperture 83 is of slightly less diameter than disk 25 so that arm BI when in normal position is effective to hold the disk in position and yet has substantially no blocking eifect thereon with respect to light rays. Arm BI is normally held in the position illustrated in Fig. 1 through registry of pin 54 in an aperture provided in the under surface of table I5. Arm 8| may be rotated about screw 82 on occasion, however, by

first pulling down the free endof the arm suf-' ficiently to disengage pin 84 from the aperture, whereby disk 25 may be readily removed and any dirt or quartz particles which may have fallen through aperture 24 cleaned away.

It will be clear from the above description that the various parts of the assembly, particularly the dilfusing disk, the pair, of focussing lenses and the reflecting mirror, are all readily removable for cleaning and replacement.

In order to further describe the invention let it be assumed that-it is desired to check for twinning and to determine the direction of the electrical'or X axes of a .Zflcut piezo-electric section and to determine the direction and sense of desired cuts. At least the upper surface of the section should first be prepared, for example, by grinding and etching or by etching alone in the instance of a sawn surface. (The etching process may utilize, for example, a 48 per cent hydrofiuoric acid for a period of twenty minutes or a Weaker solution of the acid for a longer period or other well-known fluorine etching compounds.) The section IllI to be tested is then placed in position on table I5 as shown in Figs. 1 and 2 being located over aperture 24.

The examination for twinning and the marking of the twinned regions, if any, are usually carried out at this point before the homogeneous usable regions are examined for axes. However, since the steps of the examination for twinning are more readily explainable after the orientation process has first been explained, it will be assumed for the moment that section [BI is free of twinning. It will be assumed, further, that it has been determined by previous tests, that section lilI is left-handed quartz, and has been marked with two saw notches to indicate this handedness. (As used herein, right-handedness refers to quartz which when examined in the conoscope results in expandingoptical rings when the conoscope eyepiece polarizing element is rotated clockwise while left-handedness refers to quartz which under similar conditions results in contracting optical rings.)

It might be pointed out here that if the etching time referred to above be doubled the previous test for handedness will then be unnecessary as the figures obtained (to be described later) will also indicate the handedness.

Lamp l! is now energized and the pair of lenses 55 and 45 and mirror I2 are effective to concentrate the resulting light rays through diffusing disk 25 and pin hole aperture 24. Matching arm 22 (the matching arm for use with left-handed quartz, which has been marked with two notches to indicate use of the number 2 arm) is now swung down into operative position over section I ill (as shown in Fig. 2). It might be pointed out here that arm 22, and the other matching and marking arms, are comprised, at least for a portion of their length, of flexible material so that the free end of the arm may be brought into close engagement with the upper surface of the crystal section regardless of the thickness thereof.

An aperture, I02, is provided in arm 22 near the free end thereof. Plate I53 (Fig. 1), which is provided with an aperture similar to aperture I02, is attached to the free end of arm 22 by four screws or other suitable means, being so'positioned that the aperture in'the plate is in registry with aperture 182. Matching plate I54, which is of suitable transparent material, is clamped between plate $53 and'the end of arm 22. Plate I54 is provided with three properly oriented guide lines I55, I55 and Iil'l which radiate from a common center point at intervals of Ldegrees. Plate I04 is so positioned that the common center point of the three guide lines coincides with the centerof aperture Hi2, the center of the aperture in plate 553 and pin hole 25. The assembly provided at the. free end of matching arm 22 might well be referred to therefore as a matching guide.

Now as the observer peers down through the apertured end of matching arm 22 (the arm, as stated above, having been brought down into contact with section WI) and views pin hole 24, now illuminated, he will see a figure which may be: compared, roughly, to a three-pointed star or teen equilateral. triangle in" shape and which will appear to be located between the upper sur-' face of section if and the pin hole itself. (Observance of this general type of figure is re.-

ferred to in a bound booklet by Armand de' Gramont, Recherches sur le Quartz Piezoelectric, Paris Ed. de la Rev. DOpt. 1935.) Upon detecting the figure, the observer then checks the alignment of its three lengths or axes with guide lines I85, I95 and I01. One of the many possible results of the check is indicated in Fig. 6 where it will be seen the figure is misaligned with respect to the guide lines. The observer now rotates section liH until he brings the figure into substantial coincidence with guide lines, I65, liifi and I81, this position being indicated in Fig. '7.

Now that section lGi has been orientated properly on table iii, the observer swings matching arm 22 back out of the way and brings marking arm 23'down into operative position over section Nil. The marking arm is similar, generally, to matching arm 22 but difiers therefrom in that a plate III is attached to the free end thereof by means of three screws, or other suitable fastening means. Plate iii is provided with a properly oriented Y-shaped cut-out template or marking guide H2, in which the three guide lines are again 120 degrees apart.

The observer now places the point of a pencil or other suitable marking stylus in cut-out template |I2 and marks the outline thereof on the surface of quartz section IDI. The section so marked is shown in Fig. 8. It is known to the observer that the respective X axes are parallel to the marked lines and he knows therefore that certain plates, for example, the BT out plate, may be cut parallel to any one of the lines. (The BT cut plate as disclosed in Patent 2,218,225 issued to me on October 15, 1940, is produced by so cuttingthe plate with respect to the axes of the crystal that the principalfaces of the plate are parallel to an X axis of the crystal and are oriented or inclined at an angle of approximately 49 degrees with respect to the Z axis.)

It will be understood that, in the above example, had the quartz section being examined been right-handed quartz (and marked so by one saw notch), matching arm 2! (bearing the numeral 1) would have been used instead of matching arm 22 and that, otherwise, the test would have been carried out as described. The arrangement of matching arm 21, including the assembly of the matching guide, is similar to that of matching arm 22.

Now all sections, whether of right-handedor left-handed quartz, when marked with the three radial lines as in Fig. 8, are sufiiciently marked to permit choosing the direction and sense of cuts such as the BT out referred to above. If the section, so marked, be turned so that the eye looks along any one of the three electric axes with said axis pointed toward the observer then the sense of a BT cut through the section is from.

upper left to lower right. The X axis actually selected is determined by the fact that it is often more economical from the standpoint of usable material to cut parallel toa particular one of the X axes rather than another.

In the event that the etching process be continued for a longer period, as noted above, in order that an indication of handedness may be obtained, the figure seen in the Z cut section will then be as shown either in Fig. 14 or Fig. 15. The figure of Fig. 14 is that obtained if the quartz is right-handed and shows clockwise swirls while the figure of Fig. 15 is that obtained' if the quartz is left-handed and shows counter-clockwise swirls.

If thismethodof determining the handedness of'the quartz which involves the more developed figures is to be followed, then the designations 1 and 2 on arms 2| and 22 respectively may Well be replaced by a copy of the respective figures shown in Figs. 14 and 15.

The alignment of the swirl figures with the guide lines of the matching arms is exactly the same as that described above with'reference to Figs. 6 and 7 except for the fact that we neglect the swirl portions of the figure when align- Considering now the use of the arrangement for detecting twinning as mentioned briefly above, it might be pointed out first that the figure of Figs. 6 and '7 is not identically aligned with the 'Xaxes-of the quartz section. Actually the figures are out of alignment with respect to the Xaxes by 10 degrees to 15 degrees and their sense and sense of misalignment are related to the sense of electric axes and to the handedness of the quartz as shown in Fig. 16. There is illustrated in Fig. 16, a hypothetical section of quartz in which there are four different regions exhibiting the four possible variations of electric sense and handedness that might be simultaneously exhibited in a twinned quartz section. In all cases the electrical axes of each portion are parallel to those of another portion but may be of opposite sense. As here illustrated the respective arrow point ends of the axes are the ends which would be electrically positive by tension.

The section illustrated in Fig. 17 is similar to that of Fig. 16 except that in this instance the etching process has been continued for a longer period as discussed above in order that swirl figures may be produced.

Now it will be clear that if a section such as that shown in Figs. 16 or 17 be placed on table H: with a boundary'of any two different regions directly over pin hole 24, the observer would then see parts of two different figures instead of a single figure. Twinning is easily detected if present in a section, therefore, by shifting the section about on table l5 with respect to pin hole 24 and observingrwhether or not any mixed figures occur. Actually the boundary line itself will stand out clearly and may be traced on the surface of the section with a pencil or other suitable marking stylus so that the size and shape of the several regions may be inspected for economy of cutting. A finished piezoelectric plate must be out entirely from one region and must not cros a boundary. a

When observing relatively thick, parallel faced sections for detection of twinning, it is often advantageous to check both faces of the section for twinning in order to note whether twinning boundaries go approximately straight throughthe section or whether they slant ofi to a side of the section. V

After the inspection for twinning and. after the twinned regions, if any, have been marked, all usable regions may then be tested for direction of axes and sense of cut as outlined above.

Applicant has determined that the size of the figure seen is proportional to the thickness of the crystalsection and that, when the figure is small as with verythin quartz sections, larger figures may be obtained by placing a thick piece of plate glass onthetable'under the crystal sectween l.53 and 1.56. Fig.18 illustrates a relatively thin quartz section II supported over aperture 24 of table I 5 by a relatively thick piece of plate glass I52.

If desired, section I5I may, as shown in Fig. 19,

be immersed in a liquid, which has a refractive-v index matching that of quartz, in a glass vessel I53 and the vessel positioned over the pin hole aperture in table I5. This latter method is particularly applicable in connection with sections which have only one fiat top surface; Fig. 20. shows such a section I54 in position for testing.

In addition to the obvious advantageous features of the arrangement described such as accessibility and removability of the various parts, compactness, absence of complicated moving parts, adaptability to comfortable bench use from a sitting position and adaptability to accurate use by unexperienced persons, particular attention is directed to the novel arrangement whereby the light difillsing element (disk 25) is placed under and directly adjacent to the pin hole (aperture 24) and at a distance from the light source 4I. Considerable and important advantages result from locating the light diffusing element adjacent to the pin hole rather than locating it at a distance from the aperture as has been the practice heretofore; one arrangement proposed heretofore, for example, involved utilizing the coated surface of the light source bulb itself as the diffusing element. With the light diffuser located adjacent to the pin hole as contemplated by the present invention, preparation of the under surface of the quartz section to be examined is quite unimportant and it may be ground, polished, etched or oiled as convenient and as incidental to the grinding and etching treatment usually applied to the upper surface of the section orit may be mounted in a glass dish of oil or other suitable liquid. Further, with this novel arrangement a concentrated beam of light is sent through the pin hole and strong figures are seen even under daylight working conditions.

In addition to use in determining and marking the direction of the X axes and direction and sense of cuts from a Z cut section, the arrangement of the present invention may also be used for observing twinning and for determining and marking the direction of the Z or optical axis of an X cut quartz section and the sense of the angular cuts to be made from these sections. For such use a different set of matching and marking arms is used, the arms of this set being illustrated in Figs. 10, 11 and 12;.

In order to illustrate the operation of the arrangement for this latter purpose let us assume that X cut quartz section I30 be placed on table I 5 and matching arm I3I brought down into operative position thereover as shown in Fig. 11. It will be assumed that at least the upper surface of the section has first been prepared, for example, by grinding and etching or by etching alone as referred to above. Arm I3I is provided at its free end with window I32 of suitable transparent material; two parallel lines I33 and I34 are provided on this transparent material. This arrangement may well be referred to as a matching guide.

Now as the operator, using the testing arrangement in the same general manner as described in detail above, peers down through window I32 he will see a figure in section I30. The figure in this instance, however, will not be the same as those described above as seen in the Z cut section but will be similar to one of the four figures I35, I 36, I 31, MI, which are reproduced on matching arm I3I as guides.

It will be noticed that, while the parallelogram figures I35 and I31 and the H figures I35 and MI, respectively, are of generally similar outline, they are mirror images and are easily distinguishable one from the other. Figures I36 and MI are easily distinguishable due to the fact that the longer angular toes extend in respectively differentdirections as do the respective pairs of tilted parallel lines while figures I35 and I3! are distinguishable due to the fact that the respective high sides are oppositely positioned. It might be noted that such figures are obtained, for example, by etching in 48 per cent hydrofluoric acid for 20 to 60 minutes, but that if the etching is continued for hours the appearance of the fi ures changes completely through addition of new lines to the figures as well as through loss of some of the lines here shown.

Assuming now that a figure similar to either I35 0r I36 is seen, the two arrows provided on matching arm I3I indicate that the right-hand marking arm, i. e., marking arm I42, (Fig. 12) should be used in this particular case. The two longer, vertical sides of the figure are first lined up with lines I 33 and I34 of matching arm I3I by rotating crystal section I30 after which matching arm I3 I is swung back out of the way and marking arm I42 is brought down into operative position over crystal section I30 as shown in Fig. 12.

Using edge I43 of the marking arm as a guide a line is then marked on crystal section I30 with a pencil or other suitable marking stylus. This line (line I44 of Fig. 13) indicates the direction of the optic or Z axis of the quartz crystal. Using cut-out portion I45 as a template or markin guide a second line (line I46 of Fig.13) may be drawn on the crystal section, this line indicating the sense, or direction, of the angular cuts to be made from the section. The angle of the cut-out portion provided on the matching arm will be in accordance with the particular cut of the plates to be produced from the section. In the present instance it is assumed that plates of the type commonly referred to as BT cuts are to be produced; this plate as mentioned above is disclosed in Patent 2,218,225 issued to me on October 15, 1940, and is produced by so cutting the plate with respect to the axes of the mother crystal that the principal faces of the plate are parallel to an X axis of the crystal and are orientated or inclined at an angle of approximately -49 degrees with respect to the Z axis. The angular position of cut-out portion I45 as well as that of the cut-out portion provided in matching arm M! (Fig. 10) are established accordingly. It will be understood that for other standard cuts, wherein the orientation differs from that of the BT cut referred to, the angular position of the cut-out portions may be varied accordin ly, or the orientation may even be determined from the above marking.

It will be understood that, in the above instance, had either figure I3I or 'I4I been seen in section I 30, then the left-hand marking arm (arm I41, Fig. 10) would have been used as the arrows associated with figures I31 and MI point to the left.

If section I30 had been twinned (a fact which, of course, would have been previously determined as in the instance of section IIII) parts of the two figures shown in Fig. 11 would have been joined. Fig. 21 illustrates the actual relationship of four possible figures to the four possible conditions of twinning in an X out section 163. It will be observed that the respective sides of the H shaped figures are misaligned with respect to the Z axis by 4 degrees and that the respective sides of the parallelogram figures are misaligned with respect to the Z axis by 7 degrees; the average, degrees, is theinclination of the Z axis guide line I43 of arm I42 (Fig. 12) and the similar guide line of arm M! (Fig. 10). It might be noted at thispoint also that in an X cut section, such as I63, when one surface is electrically positive the opposite surface is negative(-)-, and that theH shaped figure always occurs with the positive face and the parallelogram-shaped figure always occurs with the negative face as in Fig. 21.

It will be clear from Fig. 21 and from previous description above that the embodiment of Figs. 10, 11 and 12'may be used for detecting and marking optical twinning and electrical sense of X cut quartz sections as well as direction and sense of cut.

While certain specific embodiments of the invention have been selected for illustration and detailed description, the invention is not, of course, limite'd'in its application to these embodiments. For example quartz crystal cuts other than those specifically referred to may be examined as well as crystalline materials other than quartz. Further, the arrangement might be used in connection with study of the surface texture of manufactured transparent articles to determine proper surfacing operations. In applying the method to unknown cuts of crystals it is necessary only to determine the type-of figure that is given by the particular specimen, to relate this figure to the predetermined crystallographic axes of the 1 specimen and, with this knowledge, to then examine figures? in other specimens to determine the axial relations The surface of the specimen may be prepared in other ways than by etching, for example, it has been found that good figures maybe obtained if the surface of the specimen is prepared by grinding. The arrangements contemplated may be used for examination of any type of section,'bar, slab, wafer, plate or other shape of crystalline specimen (so long as it has at' least one plane surface) of any orientation with respect to the crystallographic axis;

for'determining the direction and sense of crystallographic axes or preferred cuts or for detecttive of the invention and not as restrictive thereof. Whatis claimed is:

H A device for testing a portion of quartz cut from a quartz mother crystal and having at least one .plane surface which has'been etched comprising a housing. a hinged table movable between an .openand a closed position and acting when in closed position as a cover for a portion .of 'said housing and having an aperture of pin hole magnitude therein, said housingbeing substantially light impervious when said table is in closed position exceptwith respect totheaperture, a light source within the said housing, a light difiuser disk, means for so removably mountingsaid disk on said table that it is adjacent to the aperture and is within said housing and is in the path of li ht from said light source when said table is in closed position and is readily removable for cleaning and replacement when said table is in open position, said disk mounting means comprising a rotatable arm having an aperture therein of slightly less diameter than said disk whereby to support said disk and yet expose substantially the'entire surface of said disk to light from said light source, means comprising a plurality of lenses and'a reflecting mirror for concentrating light from said source through said light diiiusing element and the aperture, means for so supporting said lenses 'within saidhousing that they are readily removable as a unit for cleaning or replacement when said table is in open'position, and means for so supporting said mirror Within said housing GERALD w. WILLARD.

REFERENCES" CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number .Name Date 2,264,380 Hawk. Dec. 2, 1941 2,235,590 Rockwell Mar. 18, 1941 2,167,884 Gartlandu Aug. 1, 1939 

